If you want to play Forza 5, you’re going to have to connect to the internet at first.  So even if you buy your game on disc, you’re still need to download the rest of the game in order to play it. This isn’t DLC but actual game content that’s needed to play the game. Luckily,  Forza 5 only needs to connect to the internet one time and once that is done, you’ll be able to play off-line. 

“So when you first boot up the game, we’re going to ask you to log in. And when you log in you’re going to get the Drivatars and you’re also going to get a whole bunch of content: tracks and cars. Our production schedule is such that we are putting them in as late as possible and that means making them free as downloadable content on Day One.”